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Isolated electrons and muons in events with missing
transverse momentum at HERA
V. Andreev, B. Andrieu, T. Anthonis, A. Astvatsatourov, A. Babaev, J. Bahr,
P. Baranov, E. Barrelet, W. Bartel, S. Baumgartner, et al.
To cite this version:
DESY 02-224 ISSN 0418-9833 January 2003
Isolated electrons and muons in events with missing
transverse momentum at HERA
H1 Collaboration
Abstract
A search for events with a high energy isolated electron or muon and missing transverse momentum has been performed at the electron–proton collider HERA using an integrated luminosity of13.6 pb−1 ine−p scattering and 104.7 pb−1 ine+p scattering. Within the Standard Model such events are expected to be mainly due to W boson production with subsequent leptonic decay. Ine−p interactions one event is observed in the electron chan-nel and none in the muon chanchan-nel, consistent with the expectation of the Standard Model. In thee+p data a total of 18 events are seen in the electron and muon channels compared to an expectation of12.4 ± 1.7 dominated by W production (9.4 ± 1.6). Whilst the overall observed number of events is broadly in agreement with the number predicted by the Stan-dard Model, there is an excess of events with transverse momentum of the hadronic system greater than 25 GeV with 10 events found compared to 2.9 ± 0.5 expected. The results are used to determine the cross section for events with an isolated electron or muon and missing transverse momentum.
V. Andreev24, B. Andrieu27, T. Anthonis4, A. Astvatsatourov35, A. Babaev23, J. B¨ahr35, P. Baranov24, E. Barrelet28, W. Bartel10, S. Baumgartner36, J. Becker37, M. Beckingham21, A. Beglarian34, O. Behnke13, A. Belousov24, Ch. Berger1, T. Berndt14, J.C. Bizot26, J. B¨ohme10, V. Boudry27, J. Bracinik25, W. Braunschweig1, V. Brisson26, H.-B. Br¨oker2, D.P. Brown10, D. Bruncko16, F.W. B¨usser11, A. Bunyatyan12,34, A. Burrage18, G. Buschhorn25, L. Bystritskaya23, A.J. Campbell10, S. Caron1, F. Cassol-Brunner22, V. Chekelian25,
D. Clarke5, C. Collard4, J.G. Contreras7,41, Y.R. Coppens3, J.A. Coughlan5, M.-C. Cousinou22, B.E. Cox21, G. Cozzika9, J. Cvach29, J.B. Dainton18, W.D. Dau15, K. Daum33,39,
M. Davidsson20, B. Delcourt26, N. Delerue22, R. Demirchyan34, A. De Roeck10,43, E.A. De Wolf4, C. Diaconu22, J. Dingfelder13, P. Dixon19, V. Dodonov12, J.D. Dowell3, A. Dubak25, C. Duprel2, G. Eckerlin10, D. Eckstein35, V. Efremenko23, S. Egli32, R. Eichler32, F. Eisele13, M. Ellerbrock13, E. Elsen10, M. Erdmann10,40,e, W. Erdmann36, P.J.W. Faulkner3, L. Favart4, A. Fedotov23, R. Felst10, J. Ferencei10, S. Ferron27, M. Fleischer10,
P. Fleischmann10, Y.H. Fleming3, G. Flucke10, G. Fl¨ugge2, A. Fomenko24, I. Foresti37, J. Form´anek30, G. Franke10, G. Frising1, E. Gabathuler18, K. Gabathuler32, J. Garvey3, J. Gassner32, J. Gayler10, R. Gerhards10, C. Gerlich13, S. Ghazaryan4,34, L. Goerlich6, N. Gogitidze24, S. Gorbounov35, C. Grab36, V. Grabski34, H. Gr¨assler2, T. Greenshaw18, G. Grindhammer25, D. Haidt10, L. Hajduk6, J. Haller13, B. Heinemann18, G. Heinzelmann11, R.C.W. Henderson17, H. Henschel35, O. Henshaw3, R. Heremans4, G. Herrera7,44,
I. Herynek29, M. Hildebrandt37, M. Hilgers36, K.H. Hiller35, J. Hladk´y29, P. H¨oting2, D. Hoffmann22, R. Horisberger32, A. Hovhannisyan34, M. Ibbotson21, C¸ . ˙Is¸sever7, M. Jacquet26, M. Jaffre26, L. Janauschek25, X. Janssen4, V. Jemanov11, L. J¨onsson20, C. Johnson3, D.P. Johnson4, M.A.S. Jones18, H. Jung20,10, D. Kant19, M. Kapichine8,
M. Karlsson20, O. Karschnick11, J. Katzy10, F. Keil14, N. Keller37, J. Kennedy18, I.R. Kenyon3, C. Kiesling25, P. Kjellberg20, M. Klein35, C. Kleinwort10, T. Kluge1, G. Knies10, B. Koblitz25, S.D. Kolya21, V. Korbel10, P. Kostka35, R. Koutouev12, A. Koutov8, J. Kroseberg37,
K. Kr¨uger10, J. Kueckens10, T. Kuhr11, M.P.J. Landon19, W. Lange35, T. Laˇstoviˇcka35,30, P. Laycock18, A. Lebedev24, B. Leißner1, R. Lemrani10, V. Lendermann10, S. Levonian10, B. List36, E. Lobodzinska10,6, B. Lobodzinski6,10, N. Loktionova24, V. Lubimov23, S. L¨uders37, D. L¨uke7,10, L. Lytkin12, N. Malden21, E. Malinovski24, S. Mangano36, P. Marage4, J. Marks13, R. Marshall21, H.-U. Martyn1, J. Martyniak6, S.J. Maxfield18, D. Meer36, A. Mehta18,
K. Meier14, A.B. Meyer11, H. Meyer33, J. Meyer10, S. Michine24, S. Mikocki6, D. Milstead18, S. Mohrdieck11, M.N. Mondragon7, F. Moreau27, A. Morozov8, J.V. Morris5, K. M¨uller37, P. Mur´ın16,42, V. Nagovizin23, B. Naroska11, J. Naumann7, Th. Naumann35, P.R. Newman3, F. Niebergall11, C. Niebuhr10, G. Nowak6, M. Nozicka30, B. Olivier10, J.E. Olsson10,
D. Ozerov23, V. Panassik8, C. Pascaud26, G.D. Patel18, M. Peez22, E. Perez9, A. Petrukhin35, J.P. Phillips18, D. Pitzl10, R. P¨oschl26, I. Potachnikova12, B. Povh12, J. Rauschenberger11, P. Reimer29, B. Reisert25, C. Risler25, E. Rizvi3, P. Robmann37, R. Roosen4, A. Rostovtsev23, S. Rusakov24, K. Rybicki6, D.P.C. Sankey5, E. Sauvan22, S. Sch¨atzel13, J. Scheins10,
M. Urban37, A. Usik24, S. Valk´ar30, A. Valk´arov´a30, C. Vall´ee22, P. Van Mechelen4, A. Vargas Trevino7, S. Vassiliev8, Y. Vazdik24, C. Veelken18, A. Vest1, A. Vichnevski8, V. Volchinski34, K. Wacker7, J. Wagner10, R. Wallny37, B. Waugh21, G. Weber11, R. Weber36, D. Wegener7, C. Werner13, N. Werner37, M. Wessels1, B. Wessling11, M. Winde35, G.-G. Winter10, Ch. Wissing7, E.-E. Woehrling3, E. W¨unsch10, A.C. Wyatt21, J. ˇZ´aˇcek30, J. Z´aleˇs´ak30, Z. Zhang26, A. Zhokin23, F. Zomer26, and M. zur Nedden25
1 I. Physikalisches Institut der RWTH, Aachen, Germanya 2 III. Physikalisches Institut der RWTH, Aachen, Germanya
3 School of Physics and Space Research, University of Birmingham, Birmingham, UKb 4 Inter-University Institute for High Energies ULB-VUB, Brussels; Universiteit Antwerpen (UIA), Antwerpen; Belgiumc
5 Rutherford Appleton Laboratory, Chilton, Didcot, UKb 6 Institute for Nuclear Physics, Cracow, Polandd
7 Institut f¨ur Physik, Universit¨at Dortmund, Dortmund, Germanya 8 Joint Institute for Nuclear Research, Dubna, Russia
9 CEA, DSM/DAPNIA, CE-Saclay, Gif-sur-Yvette, France 10DESY, Hamburg, Germany
11Institut f¨ur Experimentalphysik, Universit¨at Hamburg, Hamburg, Germanya 12Max-Planck-Institut f¨ur Kernphysik, Heidelberg, Germany
13Physikalisches Institut, Universit ¨at Heidelberg, Heidelberg, Germanya 14Kirchhoff-Institut f¨ur Physik, Universit¨at Heidelberg, Heidelberg, Germanya
15Institut f¨ur experimentelle und Angewandte Physik, Universit¨at Kiel, Kiel, Germany 16Institute of Experimental Physics, Slovak Academy of Sciences, Koˇsice, Slovak Republice,f 17School of Physics and Chemistry, University of Lancaster, Lancaster, UKb
18Department of Physics, University of Liverpool, Liverpool, UKb 19Queen Mary and Westfield College, London, UKb
20Physics Department, University of Lund, Lund, Swedeng
21Physics Department, University of Manchester, Manchester, UKb 22CPPM, CNRS/IN2P3 - Univ Mediterranee, Marseille - France 23Institute for Theoretical and Experimental Physics, Moscow, Russial 24Lebedev Physical Institute, Moscow, Russiae
25Max-Planck-Institut f¨ur Physik, M¨unchen, Germany
26LAL, Universit´e de Paris-Sud, IN2P3-CNRS, Orsay, France 27LPNHE, Ecole Polytechnique, IN2P3-CNRS, Palaiseau, France 28LPNHE, Universit´es Paris VI and VII, IN2P3-CNRS, Paris, France
29Institute of Physics, Academy of Sciences of the Czech Republic, Praha, Czech Republice,i 30Faculty of Mathematics and Physics, Charles University, Praha, Czech Republice,i
31Dipartimento di Fisica Universit `a di Roma Tre and INFN Roma 3, Roma, Italy 32Paul Scherrer Institut, Villigen, Switzerland
33Fachbereich Physik, Bergische Universit¨at Gesamthochschule Wuppertal, Wuppertal, Germany
34Yerevan Physics Institute, Yerevan, Armenia 35DESY, Zeuthen, Germany
38Also at Physics Department, National Technical University, Zografou Campus, GR-15773 Athens, Greece
39Also at Rechenzentrum, Bergische Universit¨at Gesamthochschule Wuppertal, Germany 40Also at Institut f¨ur Experimentelle Kernphysik, Universit¨at Karlsruhe, Karlsruhe, Germany 41Also at Dept. Fis. Ap. CINVESTAV, M´erida, Yucat´an, M´exicok
42Also at University of P.J. ˇSaf´arik, Koˇsice, Slovak Republic 43Also at CERN, Geneva, Switzerland
44Also at Dept. Fis. CINVESTAV, M´exico City, M´exicok
aSupported by the Bundesministerium f¨ur Bildung und Forschung, FRG, under contract numbers 05 H1 1GUA /1, 05 H1 1PAA /1, 05 H1 1PAB /9, 05 H1 1PEA /6, 05 H1 1VHA /7 and 05 H1 1VHB /5
b Supported by the UK Particle Physics and Astronomy Research Council, and formerly by the UK Science and Engineering Research Council
c Supported by FNRS-FWO-Vlaanderen, IISN-IIKW and IWT
dPartially Supported by the Polish State Committee for Scientific Research, grant no.
2P0310318 and SPUB/DESY/P03/DZ-1/99 and by the German Bundesministerium f¨ur Bildung und Forschung
eSupported by the Deutsche Forschungsgemeinschaft f Supported by VEGA SR grant no. 2/1169/2001
g Supported by the Swedish Natural Science Research Council
i Supported by the Ministry of Education of the Czech Republic under the projects INGO-LA116/2000 and LN00A006, by GAUK grant no 173/2000
j Supported by the Swiss National Science Foundation kSupported by CONACyT
1
Introduction
The HERA collaborations H1 and ZEUS have previously reported [1–3] the observation of events with an isolated high energy lepton and missing transverse momentum ine+p collisions recorded during the period 1994–1997. The dominant Standard Model (SM) contribution to this topology is realW boson production with subsequent leptonic decay. Such events can also be a signature of new phenomena beyond the Standard Model [4]. H1 has reported [2] onee−event and5 µ±events compared to expectations from the Standard Model of2.4 ± 0.5 and 0.8 ± 0.2 for thee±andµ±channels respectively, withW contributions of 1.65±0.47 (e) and 0.53±0.11 (µ). For the same data taking period ZEUS has reported [3] 3 (0) e±(µ±) events compared to an expectation of2.1 (0.8) W events and 1.1 ± 0.3 (0.7 ± 0.2) events from other processes. In the present paper a search for events with isolated electrons1or muons and missing transverse
momentum is performed in an extended phase space and with improved background rejection. The complete HERA I data sample (1994–2000) is analysed here. This corresponds to an integrated luminosity of118.4 pb−1, which represents a factor of three increase with respect to the previous published result.
This paper is organised as follows. Section 2 describes the SM processes that contribute to the signal and to the background. Section 3 describes the H1 detector and experimental con-ditions. Section 4 outlines the lepton identification criteria and the reconstruction methods for the hadronic final state. The selection requirements for the electron and muon channels are described in section 5. Studies of background processes are presented in section 6. Section 7 deals with systematic uncertainties and section 8 presents the results of the analysis includ-ing the numbers of events seen, the kinematics of the selected events and the measured cross sections. The results of a search for W production in the hadronic decay channel are given in section 9. The paper is briefly summarised in section 10.
2
Standard Model Processes
The processes within the Standard Model that are expected to lead to a final state containing an isolated electron or muon and missing transverse momentum, due to penetrating particles escaping detection in the apparatus, are described in detail in [2] and are only briefly outlined in this section. The processes are called “signal” if they produce events which contain a gen-uine isolated electron or muon and gengen-uine missing transverse momentum in the final state. The processes are defined as “background” if they contribute to the selected sample through misidentification or mismeasurement. For the background processes, a fake lepton, fake miss-ing transverse momentum or both can be reconstructed and may lead to the topology of interest. The following processes are considered.
W production: ep → eW±X or ep → νW±X (signal)
RealW production in electron proton collisions with subsequent leptonic decay W → lν, proceeding via photoproduction, is the dominant SM process that produces events with
1In this paper “electron” refers generically to both electrons and positrons. Where distinction is required the
prominent high PT isolated leptons and missing transverse momentum. W bosons are predicted to be produced mainly in resolved photon interactions, in which theW typically has small transverse momentum, whilst in direct photon interactions the W transverse momentum may be larger.
In this paper, the SM prediction for W production via ep → eW±X is calculated by using a next to leading order (NLO) Quantum Chromodynamics (QCD) calculation [5] in the framework of the EPVEC [6] event generator. Each event generated by EPVEC ac-cording to its default LO cross section is weighted by a factor dependent on the transverse momentum and rapidity of theW [7], such that the resulting cross section corresponds to the NLO calculation. The ACFGP [8] parameterisation is used for the photon structure and the CTEQ4M [9] parton distribution functions are used for the proton. The renor-malisation scale is taken to be equal to the factorisation scale and is fixed to theW mass. Final state parton showers are simulated using the PYTHIA framework [10].
The NLO corrections are found to be of the order of30% at low W transverse momentum (resolved photon interactions) and typically10% at high W transverse momentum (direct photon interactions) [5]. The NLO calculation reduces the theory error to15% (from 30% at leading order).
The charged current processep → νW±X is calculated with EPVEC [6] and found to contribute less than7% of the predicted signal cross section.
The total predictedW production cross section amounts to 1.1 pb for an electron–proton centre of mass energy of√s = 300 GeV and 1.3 pb for√s = 318 GeV.
Z production : ep → eZ(→ ν¯ν)X (signal)
A small number of signal events may be produced by Z production with subsequent decay to neutrinos. The outgoing electron from this reaction can scatter into the detector yielding the isolated lepton in the event while genuine missing transverse momentum is produced by the neutrinos. This process is calculated with the EPVEC generator and found to contribute less than3% of the predicted signal cross section.
Charged Current (CC) processes : ep → νX (background)
A CC deep inelastic event can mimic the selected topology if a particle in the hadronic fi-nal state or a radiated photon is interpreted as an isolated lepton. The generator DJANGO [11] is used to calculate this contribution to the background.
Neutral Current (NC) processes : ep → eX (background)
The scattered electron in a NC deep inelastic event yields an isolated high energy lepton, but measured missing transverse momentum can only be produced by fluctuations in the detector response or by undetected particles due to limited geometrical acceptance. The generator RAPGAP [12] is used to calculate this contribution to the background.
Photoproduction of jets : γp → X (background)
the hadronic final state is interpreted as an isolated lepton and missing transverse mo-mentum is measured due to fluctuations in the detector response or limited geometrical acceptance.
Lepton pair (LP) production : ep → e l+l−X (background)
Lepton pair production can mimic the selected topology if one lepton escapes detec-tion and measurement errors cause apparent missing momentum. The generator GRAPE 1.1 [14], based on a full calculation of electroweak diagrams, is used. The dominant con-tribution is due to photon–photon processes and is cross-checked with the LPAIR [15] generator. Internal photon conversions are also calculated. Z production and its subse-quent decay into charged leptons is also included in GRAPE. This contribution is found to be negligible.
In order to determine signal acceptances and background contributions, the detector re-sponse to events produced by the above programs is simulated in detail using a program based on GEANT [16]. The simulated events are then subjected to the same reconstruction and anal-ysis chain as the data.
3
Experimental Conditions
Results are presented for the 37.0 pb−1 of e+p data taken in 1994–1997 at an electron–proton centre of mass energy of √s = 300 GeV , the 13.6 pb−1 of e−p data (1998–1999,√s = 318 GeV) and the67.7 pb−1ofe+p data (1999–2000,√s = 318 GeV).
A detailed description of the H1 detector can be found in [17]. Only those components of particular importance to this analysis are described here.
The inner tracking system consisting of central and forward2tracking detectors (drift
cham-bers) is used to measure charged particle trajectories and to determine the interaction vertex. A solenoidal magnetic field allows the measurement of the particle transverse momenta.
Electromagnetic and hadronic final state particles are absorbed in a highly segmented Liquid Argon (LAr) calorimeter [18]. The calorimeter is5 to 8 interaction lengths deep depending on the polar angle of the particle. A lead–fibre calorimeter (SpaCal) is used to detect backward going electrons and hadrons.
The LAr calorimeter is surrounded by a superconducting coil with an iron return yoke in-strumented with streamer tubes. Tracks of muons, which penetrate beyond the calorimeter, are reconstructed from their hit pattern in the streamer tubes. The instrumented iron is also used as a backing calorimeter to measure the energy of hadrons that are not fully absorbed in the LAr calorimeter.
In the forward region of the detector a set of drift chamber layers (the forward muon system) detects muons and, together with an iron toroidal magnet, allows a momentum measurement. Around the beam pipe, the plug calorimeter measures hadronic activity at low polar angles.
2The forward direction and the positivez–axis are taken to be that of the proton beam direction. All polar
The LAr calorimeter provides the main trigger for events with high transverse momentum. The trigger efficiency is ' 98% for events with an electron which has transverse momentum above10 GeV. For events with high missing transverse momentum, determined from an imbal-ance in transverse momentum measured in the calorimeterPTcalo, the trigger efficiency is' 98% whenPTcalo> 25 GeV and is ∼ 50% when PTcalo = 12 GeV [19]. Events may also be triggered by a pattern consistent with a minimum ionising particle in the muon system in coincidence with tracks in the tracking detectors.
4
Lepton Identification and Hadronic Reconstruction
An electron candidate is defined [20] by the presence of a compact and isolated electromagnetic cluster of energy in the LAr calorimeter, with the requirement of an associated track having an extrapolated distance of closest approach to the cluster of less than 12 cm. Electrons found in regions between calorimeter modules containing large amounts of inactive material are ex-cluded [19]. The energy of the electron candidate is measured from the calorimeter cluster. The additional energy allowed within a cone of radius1 in pseudorapidity–azimuth (η–φ) space around the electron candidate is required to be less than3% of the energy attributed to the elec-tron candidate. The efficiency of elecelec-tron identification is established using NC events and is greater than98% [19].
A muon candidate is identified by a track in the forward muon system or a charged track in the inner tracking system associated with a track segment or an energy deposit in the instru-mented iron. The muon momentum is measured from the track curvature in the solenoidal or toroidal magnetic field. A muon candidate may have no more than8 GeV deposited in the LAr calorimeter in a cone of radius 0.5 in (η–φ) space associated with its track. The efficiency to identify muons is established using elastic LP events [21] and is greater than90%.
Identified leptons are characterised by the following variables, wherel represents e or µ:
• Pl
T, the transverse momentum of an identified muon or electron; • θl, the polar angle of the muon or electron.
The hadronic final state (HFS) is measured by combining calorimeter energy deposits with low momentum tracks as described in [19]. Identified isolated electrons or muons are excluded from the HFS. The calibration of the hadronic energy scale is made by comparing the transverse momentum of the precisely measured scattered electron to that of the HFS in a large NC event sample. The transverse momentum of the hadronic system is:
• PX
T , which includes all reconstructed particles apart from identified isolated leptons. The isolation of identified leptons with respect to jets or other tracks in the event is quantified using:
• their distance Djetfrom the axis of the closest hadronic jet inη–φ space. For this purpose jets, excluding identified leptons, are reconstructed using an inclusivekT algorithm [22– 24] and are required to have transverse momentum greater than 5 GeV. If there is no such jet in the event,Djet is defined with respect to the polar and azimuthal angles of the hadronic final state;
• their distance Dtrack from the closest track inη–φ space, where all tracks with a polar angle greater than10◦and transverse momentum greater than0.15 GeV are considered. The following quantities are sensitive to the presence of high energy undetected particles and/or can be used to reduce the main background contributions.
• Pcalo
T , the net transverse momentum measured from all energy deposits recorded in the calorimeter.
• Pmiss
T , the total missing transverse momentum reconstructed from all observed particles (electrons, muons and hadrons). PTmissdiffers most fromPTcalo in the case of events with muons, since they deposit little energy in the calorimeter.
• Vap
Vp , a measure of the azimuthal balance of the event. It is defined as the ratio of the anti–parallel to parallel components of the measured calorimetric transverse momentum, with respect to the direction of the calorimetric transverse momentum [19]. Events with one or more highpT particles that do not deposit much energy in the calorimeter (µ, ν) generally have low values of VVap
p . • δmiss = 2Ee−
P
iEi(1 − cos θi), where Ei andθi denote the energy and polar angle of each particle in the event detected in the main detector (θe < 176◦) andEeis the electron beam energy. For an event where only momentum in the proton direction is undetected
δmissis zero.
• ∆φl−X, the difference in azimuthal angle between the lepton and the direction of PTX. NC events typically have values of∆φl−X close to180◦.
• ζ2 e = 4E
0
eEecos2θe/2, where E
0
e is the energy of the final state electron. For NC events, where the scattered electron is identified as the isolated high transverse momentum elec-tron, ζ2
5
Selection Criteria
The published H1 observation [2] using 1994–1997 e+p data was based on the selection of a sample of events withPcalo
T > 25 GeV. This experimental cut mainly selected charged current events in a phase space where the trigger efficiency is high. In the selected events all isolated charged tracks with transverse momentum above10 GeV were identified as electrons or muons.
In the present paper thePcalo
T cut has been lowered to12 GeV, taking advantage of the im-proved understanding of trigger efficiencies with increased luminosity and more sophisticated background rejection. The analysis extends the phase space towards lower missing transverse momentum for the electron channel (PTcalo ' PTmiss) and towards lowerPTX for the muon chan-nel (PTcalo' PTX). The lepton identification has also been improved and extended in the forward direction. The increased phase space and increased luminosity allow the comparison with the SM predictions to be made differentially and with improved precision. Further details of the analysis can be found in [25, 26].
The selection criteria for both channels are summarised in table 1. The dominant back-ground in the electron channel is due to NC and CC events. To reduce the NC backback-ground, events with NC topology (azimuthally balanced, with the lepton and the hadronic system back-to-back in the transverse plane) are rejected. For low values ofPcalo
T , where the NC background is largest, a requirement onζe2 is imposed. A requirement that the lepton candidate be isolated from the hadronic final state is imposed to reject CC events. Events which have, in addition to an isolated electron, one or more isolated muons are not considered in the electron channel, but may contribute in the muon channel. The dominant backgrounds in the muon channel are in-elastic muon pair production and CC or photoproduction events which contain a reconstructed muon. The final muon sample is selected by rejecting azimuthally balanced events and events where more than one muon is observed.
Following the selection criteria described above, the overall efficiency to select SMW → eν events is 41% and to select SM W → µν events is 14%. The main difference in efficiency between the two channels is due to the cut onPcalo
T , which for muon events acts as a cut onPTX because the muon deposits little energy in the calorimeter. There is thus almost no efficiency in the muon channel for PTX < 12 GeV. For values of PTX > 25 GeV the efficiencies of the two channels are compatible at∼ 40%.
6
Background Studies
To verify that the backgrounds (see section 2) that contribute to the two channels are well under-stood, alternative event samples, each enriched in one of the important background processes, are compared with simulations. For both channels these event samples have the same basic phase space definition (θl,PTl,PTcalo) as the main analysis. It should be noted that these selec-tions do not explicitly reject signal events, which may be present in the enriched samples.
Variable Electron Muon θl 5◦ < θl < 140◦ Pl T > 10 GeV Pcalo T > 12 GeV Pmiss T > 12 GeV PX T – > 12 GeV Djet > 1.0 Dtrack > 0.5 for θe≥ 45◦ > 0.5 ζ2
l > 5000 GeV2 forPTcalo < 25 GeV –
Vap
Vp < 0.5 (< 0.15 for P
e
T < 25 GeV) < 0.5 (< 0.15 for PTcalo< 25 GeV)
∆φl−X < 160◦ < 170◦
# isolatedµ 0 1
δmiss > 5 GeV† –
†if only onee candidate is detected, which has the same charge as the beam lepton.
Table 1: Selection requirements for the electron and muon channels.
NC enriched sample
A NC dominated electron sample is selected by requiringDjet > 1.0. The events in this channel mainly contain genuine electron candidates, but with missing transverse momen-tum arising from mismeasurement.
CC enriched sample
A CC dominated sample is obtained by rejecting events with an isolated muon and apply-ing cuts to suppress the NC component. These criteria areζe2 ≥ 2500 GeV2, VVap
p ≤ 0.15,
δmiss > 5 GeV and ∆φe−X < 160◦. In this sample the missing transverse momentum is genuine, but an electron candidate is usually falsely identified.
The two samples designed to study the backgrounds in the muon channel, defined within the same phase space5◦ < θµ< 140◦,PTµ> 10 GeV and PTcalo> 12 GeV, are selected with the following additional requirements.
LP enriched sample
A sample of events predominantly from the two photon process is selected by requiring at least one isolated muon and VVap
p ≤ 0.2 to suppress photoproduction events.
CC enriched sample A sample dominated by CC events is selected by requiring VVap
p ≤
0.15 and requiring at least one muon candidate that need not be isolated. This selection
tests fake or real muons observed in events with genuine missingPT.
Example distributions of the background enriched event samples for thee+p data are shown in figure 2 for the electron channel and in figure 3 for the muon channel. Also included in the figures are the SM expectations from all processes together and the signal expectation alone. Agreement is also obtained between the data and the simulation in all distributions for thee−p data sample.
7
Systematic Uncertainties
The systematic uncertainties on quantities which influence the SM expectation and the measured cross section (see section 8.1) are presented in this section and discussed in more detail in [19,26]. The uncertainties on the signal expectation and the acceptance used in the cross section calculation are determined by varying experimental quantities by ± 1 standard deviation and recalculating the cross section or expectation. The experimental uncertainties are listed below and the corresponding variation of the cross section is given in table 2.
• Leptonic quantities
The uncertainties on theθl and theφl measurements are3 mrad and 1 mrad respectively. The electron energy scale uncertainty is3%. The muon energy scale uncertainty is 5%.
• Hadronic quantities
The uncertainties on the θ and φ measurements of the hadronic final state are both 20 mrad. The hadronic energy scale uncertainty is4%. The error on the measurement ofVVap
p is±0.02.
• Triggering / Identification
The electron finding efficiency has an uncertainty of2%. The muon finding efficiency has an error of5% in the central (θµ > 12.5◦) region and 15% in the forward (θµ < 12.5◦) region. The uncertainty on the track reconstruction efficiency is3%. The uncertainty on the trigger efficiency for the muon channel varies from 16% at PTX = 12 GeV to 2% at
PX
T > 40 GeV. • Luminosity
The luminosity measurement has an uncertainty of1.5%.
• Model
A10% uncertainty on the model dependence of the acceptance is estimated by comparing the results obtained with two further generators which produceW bosons with different kinematic distributions from those of EPVEC. The generators used are an implementation ofW production within PYTHIA and ANOTOP, an “anomalous top production” gener-ator, using the matrix elements of the complete process e + q → e + t → e + b + W as obtained from the CompHEP program [27].
the simulations and the control samples (see section 6). The uncertainties associated with lep-ton misidentification and the production of fake missing transverse momentum are included in these errors.
A theoretical uncertainty of15% is quoted for the predicted contributions from signal pro-cesses (predominantly SM W production). This is due mainly to uncertainties in the parton distribution functions and the scales at which the calculation is performed [5].
Source of systematic uncertainty Error on measured cross section
PX T < 25 GeV PTX > 25 GeV Leptonic quantities ±0.6% ±0.6% Hadronic quantities ±3.3% ±8.3% Triggering/Identification ±3.7% ±4.7% Luminosity ±1.5% ±1.5% Model Uncertainty ±10% ±10%
Table 2: Summary of experimental systematic errors on the measured cross section in two regions ofPX
T .
8
Results
For the e−p data sample one event is observed in the electron channel. The kinematics of the event are listed in table 3. No events are observed in the muon channel. This compares well to the SM expectations of1.69 ± 0.22 events in the electron channel and 0.37 ± 0.06 in the muon channel.
In thee+p data sample 10 candidate events are observed in the electron channel compared to
7.2 ± 1.2 expected from signal processes and 2.68 ± 0.49 from background sources. One
candi-date event in the electron channel is observed to contain ane−. This event was first reported and discussed in [2]. Four of the other candidate events contain ane+. The charges of the electrons in the remaining five events are unmeasured since the electrons are produced at low polar angles and they shower in material in the tracking detectors. In the muon channel8 candidate events are observed compared to 2.23 ± 0.43 expected from signal processes and 0.33 ± 0.08 from background sources. Four of the muon events observed in thee+p data sample are among those first reported and discussed in [2]. The event discussed in [1] is rejected from this analysis by the azimuthal difference (∆φµ−X) cut. Four of the events have a positively charged muon, three have a negative muon and in one event the charge is not determined.
Distributions of the selected events in lepton polar angle, azimuthal difference, transverse mass andPX
T are shown in figure 4. The lepton–neutrino transverse mass is defined as
where ~PTmiss and ~PTl are the vectors of the missing transverse momentum and isolated lepton respectively. The figure shows the electron and muon channels combined. Also included is the expectation of the Standard Model. The events generally have low values of lepton polar angle and are consistent with a flat distribution in azimuthal difference∆φl−X, in agreement with the expectation. The distribution of the events inMT is compatible with the Jacobian peak expected fromW production. The kinematics of the events with PX
T > 25 GeV are detailed in table 3. In three of the eighteen events a further electron is detected in the main detector (θe < 176◦). Taking this to be the scattered electron and assuming that there is only one neutrino in the final state and there is no initial state QED radiation, the lepton–neutrino mass Mlν can be reconstructed. All three events yield masses that are consistent with theW mass, having values of 86+7−9, 73+7−7 and 79+12−12 GeV. The observation of a second electron in these three events is compatible with the expectation from SMW production, where approximately 25% of events have a scattered electron in the acceptance range of the main detector.
Details of the event yields from the e+p data sample as a function of the transverse mo-mentum of the hadronic final state, PTX, are given in table 4 and 5 for the electron and muon channels respectively. The combined results for the electron and muon channels are given in table 6. At PX
T < 25 GeV eight events are seen, in agreement with the expectation from the Standard Model. AtPTX > 25 GeV ten events are seen, six of which have PTX > 40 GeV, where the signal expectation is very low. The probability for the SM expectation to fluctuate to the observed number of events or more is 0.10 for the fullPX
T range, 0.0015 forPTX > 25 GeV and 0.0012 for PTX > 40 GeV. The uncertainties on the SM predictions are taken into account in calculating these probabilities.
An excess is observed at PX
T > 25 GeV in both sets of e+p data. In the 1994–1997 data
4 events are observed compared to an expectation of 0.80 ± 0.14. In the 1999–2000 data 6
events are observed compared to an expectation of2.12 ± 0.36.
The method published in [2] has been applied to the 1999–2000 data sample. Using this method an excess of events is also seen at PX
T > 25 GeV in this new data sample: 5 events are observed for2.34 ± 0.29 expected. These 5 events selected by the method of the previously published analysis are also found by the analysis presented in this paper.
8.1
Cross Section
The observed number of events in thee+p data sample is corrected for acceptance and detector effects to obtain a cross section for all processes yielding genuine isolated electrons or muons and missing transverse momentum. This is defined for the kinematic region5◦ < θl < 140◦, Pl
T > 10 GeV, PTmiss > 12 GeV and Djet > 1.0 at a centre of mass energy3of √
s = 312 GeV.
The definition of isolated electrons or muons includes those from leptonic tau decay. The gen-erator EPVEC is used to calculate the detector acceptanceA for this region of phase space. The acceptance accounts for trigger and detection efficiencies and migrations. The cross section is thus
σ = (Ndata− Nbgd)
LA , (2)
where Ndata is the number of events observed, Nbgd is the number of events expected from processes treated here as background (see section 2) and L is the integrated luminosity of the data sample.
The cross section integrated over the fullPTX range is
σ = 0.31 ± 0.10 ± 0.04 pb, (3)
where the first error is statistical and the second is systematic (calculated as described in sec-tion 7).
This result is compatible with the SM signal expectation of0.237 ± 0.036 pb, dominated by the process ep → eW X, calculated at NLO [5, 7]. The small signal components from
ep → νW X and Z production are calculated with EPVEC [6] as explained in section 2. The
cross section is presented in table 7 split into the regions PX
T < 25 GeV and PTX > 25 GeV. Whilst the cross section in the low PTX region agrees within errors with the prediction, in the high PTX region it exceeds the expectation. Table 7 also includes two signal calculations in which all components are calculated at LO [5,6]. The calculation in [6] is the default calculation implemented in the event generator EPVEC. All the calculations agree within the uncertainties.
9
Search for
W Production in the Hadronic Decay Channel
Since the dominant SM process that produces events with isolated charged leptons and miss-ing transverse momentum is W production, it is interesting to search for W bosons decaying hadronically. The search for hadronicW decays is performed using events with two high trans-verse momentum jets in117.3 pb−1ofe+p and e−p data from the period 1995–2000.
Events are selected with at least two hadronic jets, reconstructed using an inclusive kT algorithm, with a transverse momentumPT greater than25 GeV for the leading jet and greater than20 GeV for the second highest PT jet. The minimumPT of any further jet considered in the event is set to5 GeV. The pseudorapidity η of each jet is restricted to the range −0.5 < η < 2.5. The dijet combination with invariant mass Mjj closest to the W mass is selected as the W candidate. The resolution of the reconstructedW mass is approximately 5 GeV. PTX is defined as the transverse momentum of the hadronic system after excluding theW candidate jets.
A cut on the missing transverse momentumPmiss
T < 20 GeV is applied to reject CC events and non–ep scattering background. NC events where the electron is misidentified as a jet are rejected [28, 29]. The final selection is made with the cuts Mjj > 70 GeV and |cosˆθ| < 0.6, where ˆθ is the decay angle in the W rest frame, with the W flight direction in the laboratory frame taken as the quantisation axis. This phase space is chosen to optimise the acceptance for W events and reduce other SM contributions. The overall selection efficiency for SM W production is 43% and is 29% forPX
T > 40 GeV.
The systematic uncertainty on the background prediction includes parton distribution func-tion uncertainties, the uncertainty on the jet energy scale and uncertainties due to the misiden-tification of an electron as a jet. In quadratic sum these give a total systematic error on the background prediction of23% [28, 30]. The SM W production rate has a theoretical error of
15%, which is added in quadrature to the experimental uncertainties, resulting in an overall
error of21%.
TheMjj distribution (without the Mjj cut) and the PTX distribution (with all cuts) of the selected data are compared to the Standard Model in figure 5. The final data selected show overall agreement with the SM expectation up to the highest PTX values. AtPTX > 25 GeV,
126 events are observed compared to 162 ± 36 expected with 5.3 ± 1.1 from W production.
The expectation is dominated by QCD multi–jet production. For PX
T > 40 GeV 27 events are observed in the data, compatible with the expectation of30.9 ± 6.7, where the W contribution amounts to 1.9 ± 0.4 events. Although there is increasing sensitivity to W production with increasingPX
T , it is at present not possible to conclude from the hadronic channel whether the observed excess of events with an isolated electron or muon with missing transverse momentum at highPTX is due toW production.
10
Summary
A search for events with isolated electrons or muons and missing transverse momentum has been performed in e+p and e−p data, using the complete HERA I (1994-2000) data sample. The selection has been optimised to increase the acceptance for W production events and it extends to lower values of hadronic transverse momentumPTX than in previous publications.
One electron event and no muon events are observed in thee−p data, consistent with the expectations of 1.69 ± 0.22 and 0.37 ± 0.06 for the electron and muon channels respectively in this relatively low luminosity data sample. In thee+p data sample 10 events are observed in the electron channel and8 in the muon channel. These events are kinematically consistent with
W production. The expected numbers of events from the Standard Model are 9.9 ± 1.3 and
2.55 ± 0.44 for the electron and muon channels respectively.
At low PTX, the number of observed events in both channels is consistent with the ex-pectation. At PX
T > 25 GeV, however, the 10 observed events exceed the SM prediction of
2.92 ± 0.49. An excess of events is observed in both the 1994–1997 and the 1999–2000 e+p data samples. The observed events are used to make a measurement of the cross section for all processes producing isolated electrons or muons and missing transverse momentum in the kinematic region studied.
In a separate search for hadronic W decays, agreement with the SM expectation is found up to the highest PX
Acknowledgements
We are grateful to the HERA machine group whose outstanding efforts have made this ex-periment possible. We thank the engineers and technicians for their work in constructing and maintaining the H1 detector, our funding agencies for financial support, the DESY technical staff for continual assistance and the DESY directorate for support and for the hospitality which they extend to the non–DESY members of the collaboration. The authors wish to thank K.P. Di-ener and M. Spira for many useful discussions and for providing the NLO calculation forW production.
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Run Event Lepton Pl
T /GeV PTX /GeV MT /GeV Mlν /GeV Charge 236176 3849 e 10.1+0.4−0.4 25.4+2.8−2.5 26.1+1.1−1.1 unmeasured 186729 702 µ+ 51+11 −17 66.7+4.9−4.9 43+13−22 + (4.0σ) 188108 5066 µ− 41.0+4.3−5.5 26.9+2.2−2.3 81.3+8.2−11 86.1+6.8−8.7 − (8.3σ) 192227 6208 µ− 73+9−12 60.5+5.5−5.4 74+20−25 − (7.0σ) 195308 16793 µ+ 60+12 −19 33.3+3.6−3.6 85+25−37 + (4.2σ) 248207 32134 e+ 32.0+0.8−0.9 42.7+3.9−4.1 62.8+1.8−1.8 + (15σ) 252020 30485 e+ 25.3+1.0 −1.0 44.3+3.6−3.6 50.6+1.9−2.0 79+12−12 + (40σ) 266336 4126 µ+ 19.7+0.7−0.8 51.5+3.8−4.0 69.2+2.4−2.6 + (26σ) 268338 70014 e+ 32.1+0.9−0.8 46.6+3.3−3.3 87.7+2.5−2.4 + (5.1σ) 270132 73115 µ 64+38−55 27.3+3.9−3.9 140+71−83 − (0.6σ) 275991 29613 e+ 37.7+1.0−1.1 28.4+5.7−5.9 74.7+2.3−2.4 + (37σ) Table 3: Kinematics and lepton charges of the events at high PX
T (> 25 GeV). The invariant massMlν is only calculated for those events with an observed scattered electron. The signifi-cance of the charge measurement in numbers of standard deviations is given in brackets after the sign. The first event listed was observed ine−p data. The rest were observed in e+p data.
Electron H1 Data SM expectation SM Signal Other SM processes
PX T < 12 GeV 5 6.40± 0.79 4.45± 0.70 1.95± 0.36 12 < PX T < 25 GeV 1 1.96± 0.27 1.45± 0.24 0.51± 0.12 25 < PX T < 40 GeV 1 0.95± 0.14 0.82± 0.13 0.13± 0.04 PX T > 40 GeV 3 0.54± 0.11 0.45± 0.11 0.09± 0.04
Table 4: Observed and predicted numbers of events in the electron channel for alle+p data. Muon H1 Data SM expectation SM Signal Other SM processes
12 < PX T < 25 GeV 2 1.11± 0.19 0.94± 0.18 0.17± 0.05 25 < PX T < 40 GeV 3 0.89± 0.14 0.77± 0.14 0.12± 0.03 PX T > 40 GeV 3 0.55± 0.12 0.51± 0.12 0.04± 0.01
Electron and Muon H1 Data SM expectation SM Signal Other SM processes PX T < 12 GeV 5 6.40± 0.79 4.45± 0.70 1.95± 0.36 12 < PX T < 25 GeV 3 3.08± 0.43 2.40± 0.40 0.68± 0.14 25 < PX T < 40 GeV 4 1.83± 0.27 1.59± 0.26 0.24± 0.06 PX T > 40 GeV 6 1.08± 0.22 0.96± 0.22 0.12± 0.04
Table 6: Observed and predicted numbers of events in the electron and muon channels combined for alle+p data. Only the electron channel contributes for PTX < 12 GeV.
Cross Section / pb
Measured SM NLO SM LO SM LO
Diener et al. Baur et al.
PX
T < 25 GeV 0.146 ± 0.081 ± 0.022 0.194 ± 0.029 0.147 ± 0.044 0.197 ± 0.059 PX
T > 25 GeV 0.164 ± 0.054 ± 0.023 0.043 ± 0.007 0.041 ± 0.012 0.049 ± 0.015
Table 7: The measured cross section for events with an isolated high energy electron or muon with missing transverse momentum. The cross sections are calculated in the kinematic region:
5◦ < θ
l < 140◦; PTl > 10 GeV; PTmiss > 12 GeV and Djet > 1.0. Also shown are the signal expectations from the Standard Model where the dominant contribution ep → eW X is calculated at next to leading order (SM NLO) [5, 7] and at leading order (SM LO) [5] and [6].
102 103 0 50 100 150
θ
e/
oEvents
H1 Data All SM processes SM error Electron Channel, NC Sample1 10 102 0 50 100 150
θ
µ/
oEvents
Muon Channel, NC Sample
(a) (b)
10 -1 1 10 102 103 104 0 20 40 60 80
P
T/ GeV
Events
P
calo/ GeV
H1 Data All SM processes SM error Signal Ndata = 1655 NSM = 1827±525Electron Channel, NC Enriched
10 -1 1 10 102 103 104 0 20 40 60 80
P
T/ GeV
Events
P
X/ GeV
Ndata = 1655 NSM = 1827±525Electron Channel, NC Enriched
10 -1 1 10 102 0 10 20 30 40 50
P
T/ GeV
Events
P
e/ GeV
Ndata = 33 NSM = 27.6±5.7Electron Channel, CC Enriched
10 -1 1 10 102 0 1 2 3 4 5
D
jetEvents
Ndata = 33 NSM = 27.6±5.7Electron Channel, CC Enriched
(c) (d)
(a) (b)
10 -3 10 -2 10 -1 1 10 102 0 10 20 30 40 50
P
T/ GeV
Events
P
miss/ GeV
H1 Data All SM processes SM error Signal Ndata = 30 NSM = 34.6±8Muon Channel, LP Enriched
10 -3 10 -2 10 -1 1 10 102 0 50 100 150
∆φ
µ-X/
oEvents
Ndata = 30 NSM = 34.6±8Muon Channel, LP Enriched
10 -2 10 -1 1 10 102 0 50 100 150
θ
µ/
oEvents
Ndata = 52 NSM = 53.2±10Muon Channel, CC Enriched
10 -2 10 -1 1 10 102 0 1 2 3 4 5
D
jetEvents
Ndata = 52 NSM = 53.2±10Muon Channel, CC Enriched
(c) (d)
(a) (b)
10 -2 10 -1 1 10 102 0 50 100
θ
lepton/
oEvents
H1 Data All SM processes SM error Signal Ndata = 18 NSM = 12.4±1.7Combined Electron and Muon
10 -2 10 -1 1 10 102 0 50 100 150
∆φ
l-X/
oEvents
Ndata = 18 NSM = 12.4±1.7Combined Electron and Muon
10 -2 10 -1 1 10 102 0 50 100 150 200
M
T/ GeV
Events
Ndata = 18 NSM = 12.4±1.7Combined Electron and Muon
10 -2 10 -1 1 10 102 0 20 40 60 80
P
T/ GeV
Events
P
X/ GeV
Ndata = 18 NSM = 12.4±1.7Combined Electron and Muon
(c) (d)
(a) (b)
